The present invention relates to the art of electric arc welding using a GMAW process and more particularly to a GMAW electric arc welder that creates a high frequency chain of current pulses to form a series of weld cycles constituting a weld process.
Electric arc welding of aluminum and aluminum alloy has created a dilemma for manufacturers to obtain a weld having a pleasing outer appearance. It was often necessary to use TIG welding. This welding process does produce a desired weld bead; however, it is costly and relatively slow compared to other types of welding. Consequently, over the years manufacturers of electric arc welders have attempted to develop an electric arc welder that would perform a pulse welding process on aluminum without the disadvantages associated with TIG welding and prior, less than satisfactory pulse welding processes. An early effort in this regard is shown in Tabata U.S. Pat. No. 4,994,646 wherein high frequency pulses are created in a series separated by a quiescent duration to control the weld puddle. This disclosure involves complex combination of various pulses having different frequencies and heights, as well as intentional period delays. This patent is incorporated by reference herein as an early attempt to use MIG welding for aluminum. This is background information relating to the present invention. In a like manner, Ueyama U.S. Pat. No. 5,667,709 is incorporated by reference herein. This patent discloses several prior art MIG welding methods for use in welding of aluminum and aluminum alloy. To overcome the alleged disadvantages of the MIG processes, this patent teaches a MAG welding process using high frequency current pulses, or groups of pulses, having different frequencies or other selected pulse characteristics. This is additional background information that need not be repeated in explaining the present invention. Another patent illustrating background technology is Lloyd U.S. Pat. No. 5,643,479. This patent illustrates a high frequency process using pulses, wherein the individual pulses have controlled pulse shapes determined by ramp up, ramp down, peak current and time durations. Of course, these pulses are formed on a fixed background current. This patent is incorporated by reference herein to show that well known technology exists for adjusting the profile, or shape of high frequency pulses. The present invention relates to a concept wherein the profile or pulse shape is changed for a purpose not contemplated by the Lloyd patent; however, the Lloyd patent is incorporated by reference herein as background technology relating to high frequency pulses for electric arc welding. The area of the individual pulses is changed by modifying ramp up, ramp down, peak current, and/or pulse amplitude. Japanese patent 58-176074 is also incorporated by reference as showing an electric arc welder apparently using high frequency output pulses with changes in the amplitude along the pulse train. All of these patents show background information regarding known technology for using high frequency current pulses for welding. The systems are highly complex and still present certain difficulties with respect to controlling the actual weld process, especially when welding aluminum.
The present invention is used to control a Power Wave electric arc welder sold by The Lincoln Electric Company of Cleveland, Ohio, and disclosed generally in Blankenship U.S. Pat. No. 5,278,390. This patent is incorporated by reference herein as illustrative of an electric arc welder having a waveform generator to control the contour, profile or shape of high frequency pulses generated to perform a pulse welding process.
In welding aluminum and aluminum alloy, substantial research and development work has been directed to the use of a pulse welding process either MIG or MAG, where high frequency pulses direct metal from aluminum weld wire to a molten metal puddle or pool on the workpiece. It is advantageous to coordinate each pulse with a molten ball of aluminum for propulsion to the workpiece. Each pulse should correspond to a given droplet of molten metal. However, such processes generate a substantial amount of heat and agitate the molten aluminum puddle formed during the welding process. To prevent this agitation of the puddles and the resultant deterioration of the appearance of the weld, the pulse welding processes have been used where there are successive delay periods or a reduction in the size of the pulses periodically. In accomplishing this objective, the frequency of the pulses is often drastically reduced. The molten metal in the puddle generally resumes its surface tension configuration where it hardens and does not produce the desired appearance obtainable by TIG welding. High frequency is designed to transfer metal with good arc stability, typically at a one drop per pulse rate, with. the drop diameter close to the wire diameter. Heat input designed for arc stability is the same beat input that controls the puddle heating, cooling, and solidification rate, and thus bead appearance and penetration profile. Heat input optimized for wire melting may not be optimized for puddle melting. Thus, prior pulse control systems sacrificed weld quality or bead appearance by employing a pulse delay or drastically changed pulse spacing. Consequently, prior art high frequency pulse welding for aluminum has not resulted in high quality outer appearance of the resulting weld bead without substantial modification of the pulses, discontinuation of the pulses or otherwise periodic disruption of the weld process, with the resultant degradation of the weld process. Consequently, there is a need for a system to create high frequency pulses that overcome difficulties experienced in electric arc welding of aluminum and aluminum alloy.
The present invention overcomes the disadvantage of prior efforts to use high frequency pulse welding for aluminum by creating a series of high frequency pulses having a frequency in the range of 50-400 Hz so that each pulse can generally correspond to a deposition globular of aluminum in the pulse welding process. This use of separate and distinct pulses for droplet transfer in the pulse welding process has the advantage of efficient transfer of molten metal and thus high deposition rate. The high frequency pulses are continuous to maximized efficiency and heat for the weld process. However, the high frequency pulses, in accordance with the invention, are modulated at a frequency scaled to the size of the weld puddle, depending upon the wire feed speed, travel speed, and plate thickness. The smaller the puddle, the higher the low frequency used for modulation. This frequency is between about xc2xc Hz to 40 Hz. By compromising xe2x80x9cone droplet per pulsexe2x80x9d design rule, individual high frequency pulse energy is modulated based upon the low frequency puddle formation. A weld bead having distinct and equally spaced ripple and uniform fusion line is obtained. While the instantaneous melt-off rate and arc length indeed alternates between high and low values at low modulation frequency, the average arc length and average melt-off rate is still maintained. By the technique of merely modulating high frequency pulses with a low frequency factor, there is no need for complex digital processing to form quiescent periods and/or complex weld profiles that cannot be coordinated with the frequency for transfer of globulars.
The shaped individual high frequency current pulses are produced by the pulse width modulator at a frequency in the range of 50-400 Hz. They are individually profiled by the wave shaper. Indeed, this profiling of the pulses creates high energy current pulses for the high energy portion of the weld cycle and low energy current pulses for the low energy portion of the weld cycle. The integrated shape is the heat energy of a pulse. Shaping of the pulses is made possible by the use of a high speed switching power supply with a wave shape to generator or wave shaper.
In one aspect of the invention, the pulses are each formed with a selected a ramp up, ramp down and peak current. These pulses have the same high frequency, and thus identical periods and are preferably constant peak currents to facilitate arc length feedback. By changing the slope of the ramp up, the energy of an individual pulse is changed. Likewise, changing the ramp down affects the energy of the pulse. The high energy pulses have a rapid ramp up and a slow ramp down. The ramp down can be exponential, as a time constant curve controlled by the wave shape generator. By maintaining the peak current and background current the same, the changes in the slope changes the energy of the pulse. During the high energy portion of the weld cycle, pulses with high energy are created by the wave shape generator. During the low energy portion of the weld cycle, pulses with low energy are created. The shape or profile of the pulses can be controlled by the above discussed shape features, or others, such as peak current time, ramp up time, ramp down time, and background current time and/or amplitude.
To shift between the high heat and low heat portions of the cycle, another aspect of the invention provides a counter for counting the pulses in one energy portion. When the preselected count number for one energy portion is reached, the wave shape generator shifts to the other portion. In this technique, one portion of a weld cycle has a certain set count number and another portion of the same cycle has a set count number. When these set count numbers are reached, there is a shift to the other portion. Consequently, the length of the weld cycle is determined by the total number of pulses constituting the two portions. In this manner, the weld cycle shifts between high and low heat or energy portions at a low frequency matching generally the natural frequency of the weld puddle. If higher heat is desired for the weld process, the count number for the high energy portion of the weld cycle is increased or the count number for the low energy portion is decreased. If a desired frequency for the total weld cycle is required, these number changes are coordinated. The selection of a count number can control the length of the weld cycle and the low frequency of the shift between portions. In accordance with a subordinate aspect of the invention, a software circuit or program is implemented wherein the number of pulses in the high energy portion is gradually increased from the start of the weld cycle. Thus, welding of aluminum commences at a low energy level. The count number of the high heat portion gradually increases so the high energy portion of the weld operation continues to increase. This increase in heat preferably occurs during the total welding operation. As an alternative, the heat increases for only a short time after the start of the weld operation. Both of these implementations have been made, but are dictated by the welding engineer of the manufacturing facility using the present invention.
In accordance with another aspect of the present invention, a standard wire feeder driven by a motor according to the level of a WFS signal is coordinated with the high and low energy or heat portions of the weld cycle in a xe2x80x9csynergisticxe2x80x9d control. During the high energy portion of the weld cycle, the level of the WFS signal is increased to have an increased wire feed speed. In a like manner, the level of the WFS signal during the low energy portion of the weld cycle is lowered to produce a lower wire feed speed. This coordinates the wire feed speed with the heat energy of the welding operation. If the energy level of a weld cycle is modulated by a factor or gain having the characteristics of a sine wave, saw tooth or AC curve, the wire feed speed is modulated accordingly to coordinate the wire feed speed with the energy or heat.
In another aspect of the present invention, there is provided a method of operating an electric arc welder of the type including a high speed switching power supply with a controller for creating high frequency current pulses through the gap between a workpiece and a welding wire advancing toward the workpiece so that the current pulses, together with background current, define a series of repeating weld cycles. This method involves shaping the pulses and background current in each of the weld cycles and forming the individual weld cycles into a pattern of pulses and background current between a high energy portion and a low energy portion, with the portions alternating at a low frequency. The high frequency of the pulses is substantially greater than the low frequency of the alternating weld cycle. This method controls the welder in a manner discussed with respect to the novel system used to control the welder. As one control feature of the method, the pulses of the individual portions are counted and the high and low energy portions are shifted according to the count number assigned to the portions. In some instances, the numbers assigned to the portions are the same; however, normally they are different. Indeed, the amount of heat created by the weld operation is dictated by the relationship of the high energy portion count number to the low energy portion count number. In accordance with an aspect of the invention, the count is controlled by an arc length feedback signal obtained by sensing the arc voltage. As the arc voltage changes, the preselected count number of one of the portions, or both of the portions, is modified to maintain the desired arc length. An aspect of the invention involves gradually decreasing the number assigned to the high energy portion from the start of the weld operation for at least several of the ending weld cycles. Thus, the weld is completed mostly at high heat and then gradually approaches a low heat level at the finish.
In accordance with another aspect of the present invention, an electric arc welder having a high speed switching power supply for creating equally spaced pulses and a background current defining a series of weld cycles has a wave shape generator or wave shaper. The output of the wave shaper defines the shape of the individual pulses and background current for each weld cycle. In this aspect of the invention, there is provided a circuit, which is generally a software program, to change the shape of the equally spaced pulses or background current in a repeating pattern. This repeating pattern is a modulation factor which is selected from a group consisting of a sine wave with a low frequency, an alternating generally square wave with a low frequency, a saw tooth curve with a low frequency, and an alternating curvilinear curve with a low frequency. This low frequency of the modulating factor or signal is generally less than about 30 Hz. In accordance with this aspect of the invention, the profile or shape of the pulses include a ramp up time, a ramp down time, and a peak current and a peak width or peak time. The peak time combined with the ramp up time and the ramp down time defines the period of the current pulse. The magnitude is also determined by the peak amplitude. By multiplying any of these pulse features by the modulation signal, the weld cycle modulates in the selected feature according to the modulation signal. In a like manner, the background current multiplied by one of the modulation signals causes the background current to fluctuate according to the signal. Thus, the low frequency modulation of the weld cycle is the modulation of the background current amplitude.
In accordance with another aspect of the invention, there is provided a welder having a high speed switching power supply and a controller for creating a weld process including high frequency current pulses through the gap between a workpiece and a welding wire advancing toward the workpiece. The high frequency current pulses define a series of weld cycles. A circuit or software program forms the cycles into a pattern of pulses between a high energy portion and a low energy portion, with the portions alternating at a low frequency. The high frequency of the pulses is substantially greater than the low frequency of the alternative portions of a weld cycle. In a accordance with this aspect of the invention, the wire feeder of the welder, which has a speed set by the value of a WFS signal, is shifted in accordance with the high and low energy portions. This aspect of the invention is generally implemented by a work point look-up table. However, the invention can be implemented by using the modulation signal wherein the wire feed signal is modulated according to the signal used to modulate the pulses constituting the repeating weld cycles. Using work point look-up tables, the frequency of the pulses during the high and low energy portions are not necessarily of the same frequency as in the preferred embodiment. This aspect of the invention is xe2x80x9csynergisticxe2x80x9d in that the wire feed speed is coordinated with the fluctuation of the welder output. Another aspect is the provision of a method for performing the operations set forth in the description of the welder.
Still a further aspect of the invention is the provision of an electric arc welder including a high speed switching power supply with a controller for creating a first and second weld process across the gap between the workpiece and a welding wire advanced toward the workpiece by a wire feeder driven at a speed set by the level of a WFS signal. The first process uses a first current wave form and a first level for the WFS signal. Likewise, the second process uses a second current wave form and a second level of the WFS signal. A software program alternates the controller between the first and second weld process at a low frequency. The weld cycle or pulses in both the first and second processes are counted. A circuit shifts from one weld process to the other weld process, when the count reaches a preselected number. The preselected number for the first and second weld processes can be either the same or different and can be adjusted during machine set up. A further feature of this aspect is that the invention involves the method of operating an arc welder for alternating between distinct weld processes in accordance with counting of the pulses in the individual weld processes.
The primary object of the present invention is the provision of an electric arc welder, and method of operating the welder, that is useful for aluminum and aluminum alloy and includes high frequency current pulses modulated by a low frequency factor to define individual repeating weld cycles.
Yet another object of the present invention is the provision of a welder and method, as defined above, which welder and method modulates the high frequency current pulses in a manner to produce heat that alternates at a low frequency during individual weld cycles of the weld process.
Still a further object of the present invention is the provision of a welder and method, as defined above, which welder and method produces a controlled ripple in a weld bead of aluminum welding operation while coordinating and controlling the heat used during the welding operation.
Still a further object of the present invention is the provision of a welder and method, as defined above, which welder and method coordinates the speed of the wire feeder with the heat created during different portions of the repeating weld cycle constituting the total welding operation.
Another object of the invention is the provision of a welder and method for performing alternately two separate pulse welding processes, whereby one process is high heat and the other process is low heat. The welder and method alternates between the two processes.
Still a further object of the present invention is the provision of a welder and method of operating a welder, which welder and method reduces porosity, increases gap tolerance and wire-to-joint offset tolerance. The action of the weld puddle is controlled during the welding operation, especially when welding aluminum or aluminum alloy.
Another object of the present invention is the provision of a welder and method of operating the welder, which welder and method optimizes the relationship between the heat used for melting the electrode and the heat used for melting the metal forming the workpiece where these two heat operations have different heat demands.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.